1. Field of the Invention
The present invention relates to a diazonium salt and a thermal recording material using the same. More particularly, the invention relates to a diazonium salt featuring good storage stability and light fixation properties with respect to light having a long wavelength of more than 400 nm and also being useful as a synthesis intermediate for azo dye, an analytical reagent and a material for thermal recording material, as well as to a thermal recording material comprising the diazonium salt and a coupler as color forming components.
2. Description of the Related Art
The diazonium salt is known as an important synthesis intermediate for azo dyes. A variety of synthesis methods for azo dyes have conventionally been known, which include, for example, a synthesis based on oxidation reaction, a synthesis based on reduction reaction, a synthesis based on substitution reaction, a synthesis based on addition reaction, a synthesis based on condensation reaction and the like, as described in Shin-Jikken Kagaku Koza (New Experimental Chemistry Course vol. 14-III, P. 1516–1534, published by Maruzen Co., LTD.). From the viewpoint of the availability of raw materials, costs, yield and the like, a synthesis based on an azo coupling reaction between a diazonium salt and a coupler, such as aniline, phenol or the like, is widely used as an industrial production method for azo dyes. Unfortunately, in such a method there is a danger of the diazonium exploding salt during the synthesis process. Hence, there has been a demand for the development of a stable diazonium that is little risking of exploding.
As disclosed in Japanese Patent Application Laid Open (JP-A) No. 11-228517, for example, the diazonium salt is used for the quantitative analysis of bilirubin which is a main component of cholocrome which is included in body fluids. Thus, the diazonium salt is considered to be an important compound in the medical and pharmaceutical fields, as well.
In general, the diazonium salt is a compound having very high chemical activity and reacts with a so-called coupler containing a phenol derivative or an active methylene group thereby readily forming an azo dye. Furthermore, the diazonium salt is also photosensitive so that the salt is decomposed by being irradiated with light, resulting in the loss of its activity. The diazonium salt therefore has long been used in a photosensitive recording material represented by diazo copies (see, for example, “Shashin Kogaku no Kiso (Hi-ginen Shashin)” [The Fundamentals of Photographic Engineering (Nonsilver Photograph)] P. 89–117 and 182–201, edited by Nihon Shashin Gakkai, published by Corona Publishing Co., Ltd. (1982)).
Moreover recently, the diazonium salt is property of being decomposed by light and losing its activity is being utilized. The diazonium salt is also being applied to recording materials that require image fixings. A typical example of such recording materials is a light-fixing type thermal recording material proposed in “Gazou Denshi Gakkaishi”, Koji Sato et al., vol. 11, No. 4, P. 290–296 (1982). This recording material comprises a recording layer that contains a diazonium salt and a coupler and which is heated and reacted based on an image signal so as to form an image. Subsequently, the resultant image is fixed by light irradiation.
In such recording materials which is the diazonium salt as a color forming component, the diazonium salt has an extremely a high chemical activity such that the diazonium salt is gradually pyrolyzed and loses its reactivity even in dark places. As a result, such recording materials have a drawback of a short shelf life. The above recording materials also have a drawback in that a diazonium salt compound remaining on a background portion that is a non-image portion is decomposed during the light fixation, thereby forming a colored decomposed product (stain) which, in turn, stains the non-image portion. Furthermore, the above recording material with the image fixed thereto is so poor in light fastness at the non-image portion that the stain on the non-image portion is grows when the recording material is placed under sunlight or a fluorescent light for an extended period of time.
Heretofore, a variety of methods to overcome the aforementioned instability of the diazonium salt have been proposed. Among the above methods one of most effective is to encapsulate the diazonium salt in microcapsules. Such micro-encapsulation achieves a notable suppression of the decomposition of the diazonium salt because the diazonium salt in the microcapsules is isolated from water and bases promoting the decomposition thereof. Accordingly, the shelf life of the recording material, which uses the above method, is dramatically improved, as described in “Denshi Shashin Gakkaishi” (Tomomasa Usami et al., vol. 26, No. 2, P. 115–125 (1987)).
Common methods for enclosing the diazonium salt in microcapsules are described in “Microcapsules” (Tomoshi Kondo, Nikkan Kogyo Shinbunsha (1970)) and “Microcapsules” (Tamotsu Kondo, et al., Sankyo Shuppan (1977)), and are as follows. First, the diazonium salt is dissolved in a hydrophobic solvent (oil phase). The resultant solution is added to an aqueous solution containing a dissolved a water-soluble polymer (aqueous phase) and then, emulsified by means of a homogenizer or the like and a monomer or a prepolymer for forming microcapsule walls is added to either the oil phase or the aqueous phase both phases so as to cause a polymerization reaction or a polymer deposition to occur at the interface between the oil phase and the aqueous phase. Thus, polymer walls of a polymer compound are formed to produce the microcapsules.
The microcapsule walls thus formed may comprise any of the various materials including crosslinked gelatins, alginates, celluloses, urea resins, urethane resins, melamine resins, nylon resins and the like.
Particularly for microcapsules which have walls formed of a material, such as a urea resin or urethane resin, having a glass transition point slightly higher than room temperature, the walls thereof are impermeable to matter at room temperature but are permeable to matter at temperatures higher than the glass transition point. Such microcapsules are called thermally responsive microcapsules and are very useful for heat sensitive recording materials.
That is, a thermal recording material comprising a thermal recording layer formed on a support, the thermal recording layer containing as main color forming components thermally responsive microcapsules which encapsale a diazonium salt, and a coupler present outside the microcapsules can stably retain the diazonium salt over an extended period of time and also can easily form a color image by heating and fix the formed image by light irradiation.
Thus, the micro-encapsulation of the diazonium salt accomplishes a dramatically improves stability of the recording materials.
Although the above method dramatically improves the stability of the thermal recording materials, the method dose not yet completely suppress the instability inherent in the diazonium salt, and falls short of ensuring adequate long-term storability of the thermal recording materials. In addition, the above method has the following problem. Namely even after printing and image fixation, when an image print is exposed to light over an extended period of time, the photodecomposed product of diazonium salt causes a photodecomposition reaction which cause the dye stain to grow. Hence, whiteness characteristics of a non-image portion (background portion) of the light-fixed image print decreases. As a result, contrast between the non-image portion and a developed color portion in the image print decreases.
In the recording materials according to the above method, the aforementioned photodecomposition reaction never occurs uniformly. It is known that various kinds of decomposition products occur depending on the ambient environment. Among dozens of kinds of such products there is one called a photodecomposition stain which has light absorptivity particularly in the visible wavelength region. If the formation of the photodecomposition stain is significant, the whiteness characteristics at the non-image portion (background portion) of the light-fixed image print and contrast between the non-image portion and the developed color portion are reduced. As a result, product quality of the recording material is seriously impaired.
However, the photodecomposition reaction of the diazonium salt is complicated and it is difficult to identify the decomposition products. Thus it is difficult to suppress the stain associated with the photodecomposition.
Hence, in recent years active studies regarding recording materials achieving long-term stability against the photodecomposition stain have been performed. For instance, JP-A No. 8-324129 has proposed a light-fixing type thermal recording material which utilizes a combination of microcapsules enclosing a light-fixing type diazonium salt and a specific hydrophobic oil, thereby achieving not only an excellent unprocessed stock storability but also such an excellent image storability that the image print is less susceptible to the degradation of the whiteness characteristics despite a long exposure to light after image formation.
Moreover, JP-A No. 11-78232 proposes a non-fixing type thermal recording material employing a novel diazonium salt directed to improving the stability of the salt itself. Specifically, a diazonium salt having a maximum light absorption in a short wavelength region of about 350 nm or less is encapsuled in microcapsules which are used in the non-fixing type thermal recording material for improving the whiteness characteristics in the background portion of the formed image and the image storability under a light having a wavelength of more than about 350 nm, which is typically represented by a fluorescent lamp.
However, depending upon the storage environments, the thermal recording material may sometimes fail to achieve adequate levels of unprocessed stock storability and image storability in the developed color portion and background portion (non-image portion) of a recorded image. At present, the thermal recording material needs to be further improved in the stability thereof.
Moreover recently, a high demand exists for reducing of image recording time, namely increasing the speed the image forming process including printing and image fixing. Particularly, the technologies achieving not only the aforementioned improvement in the stability of but also the high-speed image recording process in is a high demand of the light-fixing type thermal recording material using the diazonium salt. In order to meet such a demand, it is essential to improve the diazonium salt in the photodecomposition speed.
For these recording materials using the diazonium salt as a color forming component, it has been a common practice to irradiate the recording material with ultraviolet rays having a wavelength of about 360 nm for efficiently light fixing the image. However, the irradiation of ultraviolet rays requires a special light source and an adverse effect on the eyes is feared. In this connection, there has been a demand for a recording material using the diazonium salt and allowing for an efficient image fixation with visible light having a wavelength of more than 400 nm.
However, the recording material using the conventional diazonium salt suffers a slow and time-consuming fixing process when the diazonium salt is deactivated by the light having a wavelength of more than 400 nm. Moreover, in a case where the recording material using the conventional diazonium salt is exposed to the light for long hours in order to ensure complete fixation, products resulting from the fixing process undergo further reactions so that a developed color image may have reduced whiteness characteristics at the background portion.
In addition, the diazonium salts heretofore known in the art have yet to achieve both good light fixation performance under the source of light having the wavelength of more than 400 nm, and high storage stability (thermal stability).